Since this thread has gained a lot of attention from people all over the world, we have decided to translate it in English.
Please bear in mind that English is not my native language, and that I lack a sound technical background to translate this project with a pinpoint precision! I apologize for any mistakes or misspelling.
Thanks!
Pierre-Yves

After a short discussion with Sylvain [the webmaster] who agrees 300% with me, I will show you my new crazy home-made challenge:

My project consists in creating a 24x36mm reflex all by myself – no more, no less. I have had this project in mind for the past 25 years, but I didn’t master the technique nor did I have the knowledge necessary to complete it until now (I’m almost 42).

- The aim is not to use pieces scrapped from existing cameras, except for the shutter curtain fabric, ball bearings and screws; I have already found a solution for manufacturing the mirror, the focusing screen and even the prism.

- This camera will not be a copycat of an existing camera, but a new design that allows me to use my workshop equipped with model-making machine tools. Of course, the design will be drawing from existing solutions, such as the focal-plane shutter of the Leica II.

- I didn’t find anything like this on the web, even on Anglo-Saxon websites, and they do like DIY a lot, I will therefore be facing the teething problems.

- This project does have some difficulties , this wouldn’t otherwise be interesting. The most hindering issues for me will be the light proofness and the focal-plane shutter. As regard the light proofness, I will be bypassing this issue by drawing from the bottom-loading design of Leica, Zenit 1, Zorki… The only difference is that the bottom plate and the casing will be removable in a single piece. As regards the shutter, I scrutinized a Zenit 1 as well as a Zenit E from every possible angle to understand the operation, I have had some issues understanding it, but it now clear to me.

- Lastly, my camera will have to be reasonably sized in comparison with existing cameras. The technical level will be more or less on par with the one of the 1950s, I don’t think I’ll be able to implement an instant-return mirror, for example, but if one is too greedy from the start, the project fails and end up unfinished in a box.

Of course, I will be listening to you and taking your comments & criticisms into account.

In a project like this one, room is crucial: there isn’t a single half-millimetre to be wasted.
Even though I started industrial design using pencils & rulers, I will use more convenient tools, such as 3D Computer-aided Design (CAD).

Let’s now introduce the project itself. The camera body isn’t represented here for clarity purposes. The shutter curtains are here in the cocked position.
[Note: “dent” is the French for tooth. “25 dents” therefore means a gear with 25 teeth]

Starting from the take-up spool, there is firstly a 24-tooth gear meshing with two 20-tooth gears, which are in turn meshing with the 25-tooth gear of the film sprocket. This equals to a 24/25 gear ratio, the middle gears do not influence this ratio. Given that the film sprocket will do a complete revolution, the take-up spool 24-tooth gear will shift by one tooth every time, in order to implement the film counter. My system will be limited to 24-exposure film, because I am using 0.5-module sprockets, which are slightly larger than regular cameras, but this facilitates the calculations. If I was to use 36 exposures film, I would have needed a 39/40 ration.
E.g. for two 20-tooth gears, the gear diameter is 10mm, with a 10mm gear spacing.

Now the bottom view:
The film sprocket drives a 15-tooth gear that meshes with a 30-tooth gear, the latter being limited to a 180° rotation. Given that the ratio is 1/2, the sprocket does a full revolution, which insures that the film will advance without overlapping exposures.
The 30-tooth gear meshes with the shutter curtain drum through a 20-tooth gear, hence a (30/20)*180 = 270° rotation of the drum.
The drum has a 18mm diameter: 18*Pi*(270/360) = 42.5mm of curtain linear movement.

Components of the drum:
- axis
- 1st curtain parts
- 2nd curtain drum
- the other parts are used in the retaining tab of the 2nd curtain

The drum assembly:

More difficult now: manufacturing of the spring-loaded winding drums.
- the axis has a 4mm diameter
- the drum is an aluminium tube (8mm external diameter; 6mm internal diameter)
- the ring are 1mm thick, and I have to drill a 0.6mm hole in this millimetre to hook the spring into

All of this is assembled using Loctite. The axes aren’t cut to their final size yet.
The dark (Nylon) parts on the second winding drum will be used for binding the 1st curtain straps to the 2nd curtain drum.

For the project, I needed a dozen of 0.5-module gears, which cost 15 to 20€ apiece: this is expensive for mini gears, and I can’t buy a module milling-cutter either, for I’m not richer than Croesus.
I first manufactured a fly cutter (which sort of a milling cutter with only one tooth).
The fly cutter is hand made, using HSS 6 steel.

No difficult calculation here, but just a grinder and a plastic gear to check the shape.

This is a helicoidal hob, something like a milling cutter without teeth. You need to hold the gear axis with both hands, perpendicular to the thread, a 700 to 1000rpm lathe will do the rest. A M10 screw can be used instead of the hob.
On the left a "raw" gear, in the middle a gear lapped with a M10 screw and on the right a gear that has been lapped several minutes on the homemade hob.

In this case, I drew from the profile that looks like a rack-and-pinion shaped like a helicoid, for a 0.5-module.
The thread is therefore 0.5*Pi = 1.58, with a 1.70mm depth.
This is like making a screw, but with a 40° angle tool.

Without this tool, one could use a M10x150 screw for lapping, or even better a screw with 16 threads per inch (1.587).

In fact, what I have built is just like a leadscrew without teeth. If I would go further, like explained on the website linked above, I could make teeth, quench them and obtain a 0.5 module leadscrew that cost 500€.

I drilled this plate to check the good operation of the gears, as well as the gear spacing.

Since these gears are trial-and-error made, I think that result is good – much better in any case than the macro ability of my digital compact!

Working with the divider implies a perfect centering of the gear, I use a gauge.

I am here making the groove that is going to limit the rotation of the 30-tooth gear to 180°. I drilled a hole in the end of the groove in order to be able to adjust with a round file, if needed.
There is a very tiny ball bearing (2x5x2.5) in the center of the 30-tooth gear.

Here are some thoughts on the mirror:
- the flange-to-film of a m42 lens is 45.5mm
- 21+24.5mm = 45.5mm so the total is good, from the lens mount to the focusing screen. I didn’t use a Fresnel lens under the focusing screen, since I don’t know how it affects the distance. I can however place a convex corrective lens on top of the focusing screen – I don’t know if this will be useful.
- the circle represents the rotation of the mirror: it can’t collide with the back of the lens.
- I think that a plastic prototype will be necessary to validate the viewing system.

The project as of today: I am still working on the general architecture – that is, the way to place the different parts in a limited space. I am not working on the looks of the camera yet, aesthetics consideration will be the last part of the project.

The mirror issues completely emptied my brains! I’m going back to addressing the shutter and film transportation issues, for which I have much more precise ideas.

I made a focusing screen out of a 2mm thick glass. I rubbed this glass with a larger glass and some grinding powder.

Then, I removed the focusing screen of a Zenit E (which focusing screen also has a corrective lens effect), and replaced it with my own.
It looks a bit rough, but this improves when focusing – just like it does with mircroprisms.
Viewing through this ground glass without a corrective lens is acceptable for a homemade project; there is no light falloff. I tested both a 50mm and a 200mm, there a little bit of a luminosity loss when taking pictures inside, but not much – even though I have only tested “small” apertures of 1:3,5 (Industar) and 1:4.5 (Pentacon), it should be much better with a 1:1.8 Helios, but everything is packed right now!

I might be able to enhance the ground glass, I saw that grounding gel is available.
I don’t see the point of a correcting lens above the focusing screen for this homemade project: even on the Zenit E you can see it a little bit in the viewfinder.

When the shutter release button is depressed, the small gear at the bottom will disengage from the film sprocket, and the first curtain will fire, thus making this small gear rotate fully, and taking the same position again.

Simultaneously, pressing the shutter release button will switch a latch that will block the 2nd curtain tab.

The brown axis shown below rotates with the 1st curtain, and drives the blue parts along.
It is the dark blue part that is going to fire the 2nd curtain, by releasing the aforementioned latch.

The 2nd curtain is now released:
- 1/30 is achieved when the 1st curtain travels all the way
- 1/60 is achieved when the 1st curtain travels 1/4th of the way
- 1/60 is achieved when the 1st curtain travels 1/8th of the way
- etc.
- Bulb: the 2nd curtain is never released: only when the shutter released button stops being pressed will the latch be switched.

If you have trouble understanding, it’s normal, it took me some time too to figure out the refinements of a Leica II-like shutter.

It’s good to see that there are still people passionate about precision mechanics in these where nobody is surprised by nothing anymore; as a matter of fact, I think that we are losing our capability to be surprised – hopefully there still some aesthetics-loving camera collector like us. You want more, and I’ll give you more:

I have finished the base plate, only the corners will require some rounding. Don’t worry I will cut the axes to the right length, it’s not a porcupine camera!

I feel like I’m taking a 50 years step back (I wasn’t born by that time) and living what the prototyping engineers of the Leica might have lived; my means are identical, I just replaced the drawing board with a computer.

This tricky step consist in gluing the curtains to the winding drums (the screw that protrudes at the back is only temporary, for assembly purposes).

Another image with the top plate in place:

I tried manipulating the curtains, there is some friction, particularly for the 1st curtain; some adjustment with tailored washers will be necessary. The big central drum that winds the 2nd curtain is a bit too large diameterwise and I will certainly have to replace its Nylon components with micro bearing so as to reduce friction and to allow for the 1st curtain to fire more smoothly.

That means more fitting and tuning to come.

I have a question for you: have you heard of reflex cameras that need their mirror pulled down manually, i.e. independently from the film advance? I might chose this solution: cocking, then pulling the mirror down using a lever, then, when I press the release button the mirror goes up and the shutter is tripped; I could just pull the mirror down to use the viewfinder, without cocking the shutter or advancing the film.

I often have to create some tools, like this homemade square that I use for holding delicate parts like the film guide.

The film guide is used to assemble the top & bottom plates, I still need to create the sides.

You are going to make fun of me, but I already stamped the serial number. As you can see I limited myself to 1000 pieces. More seriously, I stamped the serial number now because this part is still sturdy, it will be weakened by later machining.

I modified the shutter parts: while it’s not obvious, I reduced the second curtain drum (here on the left hand side) from 16 to 15mm, the drum thickness is therefore only 0.5mm. This drum is now mounted on mini ball bearings. The sides of this drum, that are used to wind the 1st curtain, are also modified in such a way that the 2nd curtain once wound doesn’t touch them. The spring-loaded winding drums are also modified from 8 to 7mm, which makes them 0.5mm thick too.

In this project, there isn’t any wasted room, all parts are 1mm apart, and often 0.5mm only.

Technical progress of the day: I added a small, complicated part under the film sprocket, which first aim is to prop up the sprocket when the shutter button is being pressed. The other aim of this part is to prevent any light leaking from the chamber through the gears assembly.

I have designed a temporary plastic latch (on the bottom plate), I will make a sturdier one once its shaped is finalized.

I have drilled 4 notches in order to maintain the film pressure plate. These should have stopped 0.2mm short of the film slides, so that the film could move freely, but I made a mistake. Instead of re-engineering this whole part, I will make a 0.2mm groove in the film pressure plate.

On the top: the winding gears; the gear just above the sprocket will slide, so as to allow rewinding.

Today I tackled with the take-up spool, which is made of 4 parts and which is driven by a friction spring: it is the film sprocket that controls the film advance.
On the bottom of the picture, a film advance & spacing test, using an old film.

I tested winding up a 36 exposures film, to see if nothing jams (you can see my beautiful digital compact in the reflection).

I already know that you will ask: “Why lenses are you going to use?” I don’t have the arrogance to create a homemade lens – I might make one alright, but just for fun. The simplest way for me is to make a standard 42mm screw mount. What’s more, I have all a range of lenses in this mount.

I could have used a M42 mount from a for-parts Zenit (I have some of those), but where would be the masochist pleasure of the homemade craftsman, who doesn’t want to use any existing parts?

Now, that what you call heavy duty: 60mm tubing, 10mm thickness… I took me some time to cut this with a hand saw!

Then some work on my prehistoric-but-sturdy lathe number 1.

I then did the threading on my prehistoric-but-sturdy lathe number 1 – this one is more advanced but really old, the security standard are ancient too, one should be aware of catching his hand in the gears. I’m 42 and I still have all my fingers, knock on wood.

The ring was then cut, and by that time it was too brittle to be clamped on the divider, so I had to make-do with another cylinder. Of course, you need to use a lens to spot the right orientation of the lens once it is screw on the mount!

The mount is fixed with five M2 screws. Why 5? Well, I like it, I think it looks better than 4, and nobody can accuse me of using a Zenit mount!

Thanks to Web, there are some overseas French-speaking folks reading me on the other side of the ocean!

I started to reassemble the shutter; it does take a lot of fine tuning for everything to work correctly. During testing, when I cocked the shutter, the shutter release button became very stiff, stiffer than the one in a sand-filled Zenit! I took me a while to figure it out, but now I understand my mistake:

1) This is the first system, where everything is concentric. As a result, when the shutter is cocked, the gears push the film sprocket sideways, therefore jamming the shutter release spindle.
2) The enhancement: the film spool now rotates using a ball bearing; the force of the gears will now not jam the spindle. [note: jeu = backlash]
3) Given that the sprocket is a complicated part, I didn’t want to make a new one, so I just glued a tubular part on it using strong Loctite (the green one).

I didn’t see this problem coming, what really matters is finding a solution without breaking anything.

I don’t know if the DMflex will be finished for Christmas, but it has clicked for the first time!http://dailymotion.com/video/xav5p4_fon ... du-dm_tech
I adjusted the spring tension using nothing but intuition; I have 1/28th for the 1/30th setting, which is good enough since it is within tolerances. For the time being, these parts are only prototypes, I will need to rework some of them.
I tried the 1/30, 1/60 and 1/125 and the linearity of these speeds is correct. I can tell because I control the curtains operation on 3 spots (right, middle, center). I think I may add 1/250 as well.

I did the pale blue part on the picture above following the theoretical calculations et here is the result:
1/30 ok
1/60 = 1/90
From 1/125 on, nothing, there isn’t any space between both curtains.

I therefore added 2 holes, one before the 1/60 to try to get the 1/60, one just after to try to get the 1/125.
For the time being, I have 1/30 1/60 1/125 with a good linearity. I can drill a hole after the 1/125 to get the 1/250 speed, but I didn’t try so far. I don’t think 1/500 will work on this project. On most Zenit cameras, the 1/500 setting usually gives a slower speed, roughly 1/380 or maybe 1/400. I also noticed that the higher speeds are more accurate on rotating speed dial Zenit.

For the moment, I can find the right speeds but only using trial and error: as a result, this speed selection ring is starting to look like Swiss cheese, I will need to make a new one.

I also made a new system to lock the winding drums axis in place. It will possible to tune these drum using screws on the top plate – I know, that’s eccentric! Two simple screws lock the axes in place. By the way I don’t need anymore the two large screw on the bottom plate, which were ugly but useful!

I solved the mirror problem: it will be coupled with the shutter release.

There is a lever that is using the vertical movement of the shutter release. This lever has a set screw on its axis in order adjust the angle.

The lever that retains the mirror releases it by swithching forward.

I used a green leatherette – it could become my trade mark, who knows?

The image is of course reversed left-to-right, as you can see on this improvised cardboard finder.

More about the mirror: this isn’t an instant-return mirror. I do not know how to use a camera movement to return the mirror in the viewing position. If I don’t find a solution, I will use a manual lever to bring the mirror down. This isn’t the best solution, but at least I will be able to return the mirror into viewing position without cocking the shutter.

I finally decided to make a small lever to return the mirror in its down position. The prototype is working; I need to work on a definitive one. This way the operating mechanisms are much simpler, plus I won’t be slowing the shutter operation. According to my testing, the noise is very limited and less metallic than a good old Zenit.

The handling of this lever is ok, one can press it while winding & cocking for the next view, so it doesn’t slow the camera operations. Of course, the mirror locks up automatically when the shutter release is pressed, which is a good idea!

Today I tackled with the top of the camera. The finder will be a waist-level finder – just like looking a digital camera screen, only this one is reversed left-to-right.
I started with a aluminium block that I hollowed out. It’s only 2mm thick now, so it is quite light at 35grams.

So this is what the camera will look like, sort of a hybrid between a Zenit and an Exacta. I changed the knobs, they are now made of inox [stainless steel], I would have otherwise needed to nickel-plate the brass.

Here is the small lever that takes the mirror in the viewing position. It is not inconvenient, since you can press it while winding or even before winding, unlike some 1950s SLRs where you were “left in the dark” until you wound.

I will paint the bottom plate with the same green so that is it looks uniform. As you can see there is a tripod socket.

I assembled the shutter and painted the whole thing with black paint to avoid light reflection, too bad this hides the “beautiful mechanisms”.

The main drum and the winding drums are now installed on the bottom plate. For the time being the winding drums’ tension is hold using two M3 screws. I also made a part that hide the instant-return mirror mechanism on regular camera; here it doesn’t hide anything at all but it nevertheless useful to protect from light leaks.

The top is ready for assembly:

Both parts assembled, everything needs to fit in.

I then tested the shutter speeds using 3 control points (right, middle, left) to make sure that curtains travel at a constant speed.

It’s not bad at all. Bulb is working, 1/30 is a tad fast, 1/60 is correct, 1/125 is to slow, and so is 1/250.
It really isn’t that bad in comparison with older production SLRs. Getting the exact shutter speed isn’t in my opinion as important as having a constant curtain travel speed, which I did achieve here (not so much for the 1/250).

Today’s work: I finished all the stainless steel knobs; the shutter button even has a cable release thread. I didn’t have time for a conical thread like those on regular cameras, but a M3 thread will do just fine.
As I have neither the time nor the will to make myself an engraving machine, I printed transparent stickers on my computer. They really look good on polished steel, I fell like it gives a “pro” look.

[Reply to someone asking about the time & money investment in this project]

I don’t calculate the financial cost: this is a hobby and like any other hobby you spend some money, but not that much when you look at the “annual cost”.

As for the time, it is difficult to calculate. I spent roughly:

1/3 of the time on the blueprints & engineering
1/3 of the time actually making stuff
1/3 of the time adjusting and fine-tuning
It doesn’t take me that long to make the parts, because I think & design them in such a way that they will be easy to make. I also know my tools very well, and I know a lot of tips to save time.

One thing for sure, I will have used a lot of M2 taps & dies, these are really brittle! And when they do break it is very difficult to take them out without damaging the part you are working on. I know where to find good quality taps & dies, but they are expensive.

I finished working on the last details.

I cut the pressure plate from 3mm thick brown Plexiglass (I would have used black if I had some). The legs of the pressure plate are 0.2mm higher than the plate, so that the film can slide without scratching.

I also finished the two locking screws. It is harder than it looks, because the mobile part is 2.5mm thick and it is attached through a 1mm axis. Just imagine, drilling 1mm holes in stainless steel!
These are not quick-release locks, but rather disguised butterfly screws

The pressure plate in place: two pieces of foam act like a spring – by any means this is sufficient since there almost no room between the back of the pressure plate & the camera body.
I also added a foam light seal at the bottom of the top plate.

Yesterday morning the sun was shining bright, so I left with some Russian lenses and an Ikophot [lightmeter] and loaded an expired 100iso film from Super-U [a department store].
The pictures were taken between 9 and 11am, and then I dropped the film at a local 1-hour place.

When I came back one hour later I was shocked by what the clerk gave me back! No light leaks, no scratches, no unwanted reflection… well nothing that could give away this was taken with a homemade camera! By looking closer, I can see a small darker strip on the right side of the picture, maybe cause by a curtain. I don’t really know what causes this strip at the moment.

I scanned the prints on my Lide25 scanner; I didn’t crop or adjust anything. It’s only a test roll so don’t go looking for something artistic here, I took what the countryside had to offer in a two-hour time lap.